The invention relates to a transmission electron microscopy (TEM) sample, as well as to a method for the production of such sample.
Samples for TEM can be prepared in various ways. To be able to examine samples with TEM, these must be thinned as defined, such that transmission through them is possible. Herein the quality of the sample determines essentially the quality of the image resolution. For this purpose the sample should be set uniformly to the correspondingly desired defined thickness through an appropriate etching process. It is important that in this etching process the sample structure is not changed by the process itself. To produce such a sample, a piece of the material to be examined is mechanically sawed out of the sample body and subsequently thinned further to the necessary or desired thickness by etching in order to be able to examine the sample subsequently with TEM. The wet-chemical etching method does not lead to the desired goal in this respect. For this reason, for high-quality TEM samples, the samples are currently worked by etching with an ion beam. As the ion beam is employed, for example, an argon ion beam with a diameter of approximately 1 mm.
Samples for transmission electron microscopy can by now be prepared in various ways. A method has been described in the literature, which is referred to as xe2x80x9cwire shadow TEM cross section preparation techniquexe2x80x9d. This so-called wire shadow method is described in Ultra Microscopy 70 (1997) 23-28, published by Elsevier Science B. V. Verlag 1997 under the title xe2x80x9cOptimisation of the wire shadow TEM cross section preparation techniquexe2x80x9d by S. Senz, P. Kopperschmidt, E. Langer, H. Sieber, D. Hesse. These authors propose sawing a sample of the material to be examined from the solid body with a diamond saw. The sawed-out sample piece is rectangular, approximately 2-3 cm long and 200-300 xcexcm wide. The width of the sample piece is additionally reduced to a value of 100 xcexcm by mechanical working. In this process, the front-side face must not be damaged. The sample was, for example, cut from a semiconductor material such as silicon. After the sample has been sawed out of the sample body, placing, positioning and adhering takes place of an approximately 10 xcexcm thick wire (shadow wire) or a fiber onto the front face of the sample, which represents the original sample surface. For this purpose, on both ends of the sample body a small amount of epoxy adhesive is applied onto the surface and subsequently the fiber, which has a length of a few mm, is centered on the sample surface and wetted with the adhesive. The adhesive should herein flow by capillary forces into the interspaces between the sample body and the fiber. The adhesive is subsequently cured. This entire process must be carried out highly precisely and is difficult of execution. After the adhesive has been cured, the sample is placed into a sample holder and in a vacuum receptacle the sample regions not shadowed are eroded through ion bombardment perpendicularly to the sample surface so far that an electron-transparent crest is generated in the margin of the shadow transversely to the sample surface. Under the ion bombardment the diameter of the shadow fiber is also decreased whereby the crest assumes a wedge form. The height of the crest can be affected as a function of the ion erosion rate of the material to be prepared, of the erosion rate of the shadow wire and of its diameter. To generate a smooth surface morphology on the side walls of the crest, a homogeneous material erosion is necessary. This can be attained through the selection of a suitably high ion acceleration voltage and also an additionally oscillating relative movement of the sample with respect to the incident ion beam. As a result of this cross section preparation an electron-transparent crest is generated with a length of a few hundred xcexcm to mm. At the margin region of the crest close to the surface high-resolution, electron-optical characterizations are possible. One problem of the described technique of preparation comprises that it is very difficult to carry out. Preparation steps which determine the quality are the precise adhesion of the shadow fibers. In addition, the preparation of a sample takes several hours, which strongly reduces the economy of the process. For the reasons stated above, this form of preparation technique has so far not been widely accepted in the technical field.
The invention addresses the problem of eliminating the shortcomings of prior art. In particular a wire shadow or fiber shadow method will be realized, which makes possible realizing TEM samples at low expenditures, high reliability and good sample quality at simultaneously high economy.
The problem is solved according to the device of the invention while proceeding following the method of the invention.
According to the invention the TEM sample is cut from a solid state material such as a semiconductor, with the result of forming an elongated sample body with a front-side sample surface onto which an adhesive means, capable of flowing, is substantially applied over the entire surface. After a fiber is placed onto the adhesive, the latter is cured. When placing the fiber onto the surface, it becomes self-aligned in the longitudinal direction of the sample body on the sample surface and centered in the sample width through the occurring capillary forces. Due to the wetting of the sample surface with the adhesive, which covers substantially the entire area, the fiber can be wetted over its entire length and, with the aid of the capillary forces, can be positioned independently, such that precise placement is not necessary. On the other hand, through this type of adhesive application, structured sample surfaces are filled completely with the adhesive, such that no interspaces or voids are generated along the fiber, which would be highly negative for the quality of the TEM sample. When proceeding according to the teaching of prior art, such voids are not always entirely avoidable in the case of structured surfaces, since the adhesive is only applied at the sample ends and should become distributed with the aid of the capillary forces on the entire length between fiber and sample surface. Proceeding according to the invention is therefore especially suited for the preparation of samples of semiconductor materials such as are used in microelectronics. In this case samples are cut out of a semiconductor wafer such as for example silicon. Such a wafer is typically a few tenths mm thick and structured on the operative side and comprises differing layers through which the microelectronic structural elements are realized.
The fibers align themselves especially well without additional centering if the sample width is moreover sufficiently narrow. If the sample width is less than 20 xcexcm, the effect of the independent alignment of the fiber on the sample surface proceeds especially well. It has been found that optimum results are attained if the sample width is in the range of 10 to 20 xcexcm. The self-alignment effect of the fiber is still operative even if its diameter is slightly larger than the sample width, however, it is better if the fiber diameter is not greater than the sample width. It has been found that especially good results are obtained with a fiber diameter in the range of 5 to 20 xcexcm. It is additionally favorable if the fiber covers at least half of the adhesive. As adhesive agents are suitable flowable adhesives, which after a certain time cure by themselves or are curable. Preferred are here epoxy adhesives. Especially suitable fiber materials are carbon or silicon carbide, which are produced in the form of a wire with a substantially circular cross section.
The sample prepared in this manner is now thinned by ion beam etching in a further treatment step. The direction of incidence of the ion beam is substantially perpendicular to the sample surface and directed onto the fiber. Through the shadow effect of the fiber a wedge-form sample is generated, which becomes transparent to electrons at the thinnest site. It is herein advantageous to move the ion beam back and forth through a specific angle relative to the sample within the plane perpendicular to the sample surface through the fiber. Etching takes place until the desired electron transparency for the TEM in the transverse direction to the crest has been reached. It is not absolutely necessary that the fiber is eliminated through the etching. It can remain or be etched away entirely or partially. It is advantageous to etch until the fiber has disappeared in one location. In this case, slight thickness differences of the sample crest are formed, and it is possible to select a favorable examination site for TEM.
The advantages of the method according to the invention comprise that a target preparation is readily possible. This means that the desired location for the examination of the sample can be precisely defined, for example with an accuracy in the pm range. In addition, the thin sample leads to low etching times in the range of one hour and to total preparation times of approximately two hours making the sample preparation extremely economical. According to prior art, for making samples for the target preparation, a time expenditure in the range of 3 to 10 hours is necessary. According to the present method according to the invention large clean electron-transparent sample areas can be produced which are several mm long. This means that the original structure of the material in the sample to be examined is changed only minimally or not at all, thus a low degree of surface amorphization or a small disturbance depth occurs in the material. This makes possible high fidelity to the original of the image in the TEM of the material to be examined.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.